1. Field of the Invention
The field of the invention is that of digital transmission at very long distances (several thousands of kilometers) on optical fibers, in systems using line optical amplification.
In very long distance systems such as these, one of the main factors limiting the bit rate is the distortion induced by the transmission fiber.
Indeed, while these distortions can be generally overlooked in standard optical fiber systems (providing links of the order of some hundreds of kilometers), they become highly disturbing factors in long-distance transmission systems.
The invention relates to a system of transmission on an optical fiber line enabling compensation, on line, for this distortion induced by the transmission line in the case of transmission lines having a length of at least a thousand kilometers such as those used for transoceanic links.
2. Description of the Prior Art
The distortion provided by the transmission fiber arises out of the combined existence of two phenomena that occur in monomode fibers: chromatic dispersion and non-linear effects.
The first phenomenon is that of chromatic dispersion. This phenomenon arises out of the frequency dependence of the refractive index of silica. It results in,propagation times that differ according to the operating wavelength. Generally, the chromatic dispersion tends to widen the pulses of the digital trains and, therefore, to create inter-symbol interference.
In the commonly used fibers, the chromatic dispersion is zero around 1.3 .mu.m and takes a positive value of about 17 ps/nm/km around 1.55 .mu.m. It is also possible to use fibers with offset dispersion that are designed to have zero chromatic dispersion in the region of 1.55 .mu.m.
Very long distance systems (covering distances of several thousands of kilometers) work at 1.55.sub.-- m. The excessive value of the chromatic dispersion, at this wavelength, of the commonly used fibers rules out their use, and in this case therefore it will be fibers with offset dispersion that are used.
It must be noted that the effect of distortion by chromatic dispersion depends greatly on the spectral components of the pulses: if a pulse has optical phase variations that are positive at its beginning and negative at its end, it will be greatly widened by a positive chromatic dispersion. The converse is true for negative dispersions.
Finally, the chromatic dispersion induces distortions that are independent of the optical power.
The second phenomenon relates to non-linear effects. The most important non-linear effect in a fiber is the Kerr effect. This effect, which is described for example in KW. Blow and J. J. Doran, "Non-linear effects in optical fibers and fiber devices" (IEEE Proceedings, June 1987, pp. 138-144), expresses a linear dependence of silica with respect to the optical power.
This effect is very low in the usual fields of operation of optical systems (at distances smaller than about 400 km and at power values lower than about 10 mW), but becomes non-negligible for very high power values (of the order of 1 W) or for very great distances of propagation at moderate power values (some hundreds of kilometers in a system using periodic amplification).
When there is no chromatic dispersion, the Kerr effect induces a phase self-modulation of the optical pulse: the phase diminishes at the start of the pulse and then increases at its end, proportionally to the optical power. This induces a widening of the optical spectrum and a spectral composition that promotes a major degree of widening for negative chromatic dispersion.
In long-distance amplified systems, the distortion provided by the transmission fiber should therefore be considered to be the combination of the chromatic dispersion (the first phenomenon) and of the non-linear effects (second phenomenon).
The combination of these two effects may be described by a non-linear equation with partial derivatives in distance and in time known as Schrodinger's non-linear equation, the resolution of which is discussed notably in G. Agrawal, "Non-linear Fiber Optics", Academic Press.
The numerical and analytic study of this equation show that there are two types of behavior that are qualitatively very different depending on the sign of the chromatic dispersion (D) and the shape of the pulses.
The first type of behavior corresponds to D&gt;0. In this case, the Kerr effect and the chromatic dispersion have opposite effects on the transmitted pulses. In general, this leads to phenomena known as "modulation instability" phenomena: the pulses "burst" into very short pulses after 1,000 to 2,000 km and the optical spectrum widens considerably. This may lead to problems relating to the optical passband. There also exist, in this type of behavior, pulses of a particular type known as solitons which, being defined precisely by their temporal and spectral shape and their peak power, have the property of getting propagated without any modification of shape.
The second type of behavior corresponds to D&lt;0. In this case, there is no modulation instability and the pulses retain a certain degree of integrity while the spectrum widens monotonically during the propagation in keeping reasonable widths. However, the pulses widen greatly in the course of time. This creates inter-symbol interference. This interference, which depends increasingly on the absolute value of the chromatic dispersion, thus greatly restricts the acceptable range of chromatic dispersion in this field.
Already, for very long distance (6,000 to 9,000 kin) amplified underwater systems, working at a bit rate of 5 Gbit/s,, the above-mentioned effects are major effects and call for special precautions as regards the signal wavelength, the output power of the amplifiers and the choice or arrangement of the line fibers according to their chromatic dispersion. The range of operation chosen is then that of the quasi-zero negative chromatic dispersion (D&lt;0) with a so-called NRZ modulation format (intensity modulation, direct detection).
Consequently, for the designing of systems having a similar length but an even higher bit rate (10 Gbit/s and more), the above-mentioned propagation effects become crucial and highly restrictive. The designers are then faced with an ad hoc choice over the field of chromatic dispersion: namely chromatic dispersion that is either negative or positive.
Indeed, when the chromatic dispersion is almost zero and slightly negative, the field of operation in terms of wavelength is further reduced with a certainty that the manufacturing constraints will be more severe as regards the line and the transmission/reception elements.
For a chromatic dispersion value such as this that is almost zero, all the known methods of compensation for distortion introduced by the transmission line implement either compensation at transmission or compensation at reception. All these known methods therefore have the major drawback of not allowing the temporal and spectral reshaping of the pulses. Consequently, these known methods do not make it possible to prevent an exaggerated widening of the pulses which prompts inter-symbol interference.
In any case, it would appear to be extremely difficult to obtain a bit rate of over 10 Gbit/s in the current state of the art.
Furthermore, should the chromatic dispersion be positive and should the pulses be solitons, these solitons have the advantage of not getting deformed during propagation and are therefore immune to propagation distortions. However, these solitons-type pulses get mixed non-linearly with the noise put out by the amplifier. This produces a temporal jitter at reception (known as a Gordon-Haus jitter) which is a source of levels of error. If this jitter alone is considered, the limitations on bit rate in systems using solitons at very long distances (6,000 to 9,000 km) are in the range of 5 Gbit/s,. In order to increase the bit rate despite this jitter, several approaches have been proposed.
Two first known approaches are based on multiplexing techniques (wavelength multiplexing in one approach and polarization multiplexing in the other).
Two other known approaches, which seek to reduce the jitter itself in order to increase the maximum monochannel bit rate, implement an on-line control. One approach implements a control by filtering which consists in placing relatively narrow optical filters on the line. The other approach implements a control by modulation in which an on-line optical remodulation is carried out on the train of solitons at the clock frequency. This approach therefore implies the use of high frequency modulators in the line.
The latter two methods have drawbacks. For the on-line filtering, the reduction of jitter depends on the spectral narrowness of the filter. Now, the narrowness of this filter cannot be increased to an exaggerated extent for the necessary extra gain associated therewith then leads to a very substantial increase in the noise given by the amplifiers. A so-called "sliding filter" has been proposed. This consists in making the center frequency of the filters slide all along the line. This method gives good results in the laboratory but appears to be difficult to implement in a system because the frequency shift is very low (some hundreds of MHz) as compared with the width of the filter (about a hundred GHz) and is therefore very difficult to achieve on an industrial scale. For the on-line modulation, the restriction arises rather out of the on-line modulator component and its electronic circuitry which has to work at very high speeds and requires electrical supply on the line. Finally, unlike filtering, modulation is not compatible with wavelength multiplexing.
The invention is aimed notably at overcoming these different drawbacks of the prior art.
More specifically, an aim of the invention is to provide a system capable of compensating for the distortions induced by a very long distance transmission line on optical fibers with on-line optical amplification.
Another aim of the invention is to provide a system such as this that is simple to implement, costs little, is reliable and requires neither any electrical supply on the line nor any precise setting of electronic modules.
It is also an object of the invention to provide a system such as this that makes it possible to envisage high monochannel bit rates (of over 10 Gbits/s) and is compatible with wavelength multiplexing.
Another aim of the invention is to provide a system such as this that can be used to stabilize the level of noise on the "0"s and the "1"s.
A complementary aim of the invention is to provide a system such as this that can be implemented with fibers that have almost zero (and below zero) chromatic dispersion and are associated with solitons as well as with fibers having positive chromatic dispersion.
Another aim of the invention is to provide a system such as this which, in the case of a transmission of solitons, has a very high rate of jitter reduction.
Another aim of the invention is to provide a system such as this making it possible, in the case of a positive chromatic dispersion, to obtain a temporal and spectral reshaping of the light pulses conveying the information.